Optical taps are used in many optical networks in such applications as power balancing, signal monitoring or feedback control for amplification or attenuation purposes. An optical tap operates by diverting or ‘tapping’ a small, predetermined portion of the signal power that can then be measured to determine the power in the main signal without appreciably attenuating that signal. Although optical taps are typically used to tap a small portion of an input signal, they may tap more than 50% of an input signal.
The requirements to an optical tap are generally low insertion loss for the signal channel and minimal wavelength dependent loss (WDL) and polarization dependent loss (PDL) for the tapped channel.
Polarization dependent loss is defined to be a measure of peak-to-peak difference in transmission of an optical component or system with respect to all possible states of polarization. It is the ratio of the maximum and the minimum of all possible states of transmission of an optical device with respect to all polarization states.
PDLdb=10*Log (Pmax/Pmin)
By way of example, a specification for an optical tap is:
i.Insertion loss (Signal Channel)<0.2 dBii.PDL (tap channel)<0.2 dBiii.Wavelength uniformity (tap channel)<0.2 dBiv.Tap ratio2–5%
One of the most common approaches to producing an optical tap in waveguide devices is using a directional coupler 10 as shown in FIG. 1a. 
The operation of a directional coupler is based on coupled mode theory and is well described in the literature. Directional couplers have been disclosed by Derwyn C. Johnson and Kenneth O. Hill in U.S. Pat. Nos. 4,291,940, 4,779,945, 4,900,119, 5,004,316, and 5,054,874 incorporated herein by reference. In essence, two waveguides are brought into close proximity for a predetermined length such that light from one of the waveguides couples to the adjacent waveguide. The amount of light which couples into the adjacent waveguide is determined by several factors including but not exclusive to the refractive index profile of the waveguides, the separation of the waveguides and the length of the coupling region. The plot of FIG. 1b shows the dependence of the coupling efficiency from waveguide 1 to waveguide 2 as a function of the coupling length for both polarization modes. The coupling is sinusoidal with the coupling length, with slightly different periods for the two polarizations Coupled mode theory determines that the coupling between waveguides will obey a sinusoidal dependence on directional coupler length as indicated in FIG. 1b. 
The directional coupler 10 provides a suitable building block for an optical tap since, for a given design, adjusting the length of the coupler can be used to change the amount of light which is coupled or ‘tapped’ from the main signal waveguide. The excess loss of directional couplers is usually very low and so the requirements of low insertion loss on the signal channel and accurate control of the tap ratio can be achieved. However, for silica-on-silicon based optical taps, that is, waveguide devices that comprise doped silica overlying a silicon substrate, low tap PDL is difficult to achieve.
For all-silica optical fiber based taps, the thermal expansion mismatch between the core and cladding material is low (typically <0.5 ppm). Although fiber drawing is a high temperature process, the low coefficient of thermal expansion (CTE) mismatch means that thermal stresses generated in the materials are small with respect to planar silica-on-silicon devices and so stress induced birefringence is equally small in such optical fiber. In contrast, silica-on-silicon devices such as planar waveguides have very large CTE mismatches between core, cladding, and substrate materials resulting from the common necessity to deposit final overcladding layers with a much lower softening temperature than the already etched core layers. Thermal stresses are induced in the device during processing which lead to stress induced birefringence in the waveguiding region. The polarization dependence of directional coupler based taps is well known to be caused by stress induced birefringence leading to a difference in coupling lengths for the two polarizations (PDCR—polarization dependent coupling ratio). Coupling of TM modes is enhanced leading to a shorter coupling length for the TM polarization as shown in FIG. 1b, which schematically indicates the sinusoidal variation of power in the two waveguides as a function of the coupling region length. The difference in coupling strength means that the TE curve is stretched in the length axis with respect to the TM curve. Even though the tap is designed for very small coupling ratios, the real data shows that the PDL resulting from PDCR is still at an unacceptable level even for a tap ratio of −15 dB.
For silica-on-silicon devices one of the methods to achieve a polarization independent tap function is to compensate for the imbalance or higher coupling ratio of TM mode in the tapped output of the directional coupler using additional waveguide devices with opposite optical characteristics, for example providing high loss for TM mode. Such a scheme is disclosed in U.S. Pat. No. 5,539,850 in the names of Henry et al., where compensation was achieved using a second directional coupler that again preferentially couples the TM mode. This configuration is shown in FIG. 2 where a first directional coupler 20 having an input trunk waveguide 22 is coupled to a first branch arm 24 which itself serves as a trunk arm from which a second compensating coupler 26 is disposed to couple into an unused branch 28. The tap waveguide 27 has an output that is compensated. Typically, this compensation is accomplished by choosing a short and a long coupling length such that the transmissions are on opposites sides of a sinusoidal peak as in FIG. 1b, where the differences in transmission with polarization are of opposite signs.
It is an object of this invention to provide a relatively inexpensive, combination of waveguide coupler and bend components that make up an optical tap, said bend component being operative to substantially compensate for polarization dependent loss that the tap would otherwise have suffered between its input and output ports, absent the bend component.